1. Field of the Invention
The present invention relates to methods and apparatus for testing and monitoring, and more particularly, to means for detecting and monitoring scour in bridge piers and similar structures.
2. Brief Description of the Prior Art
Bridge scour is a severe problem that costs millions of dollars in terms of damage, loss of life and required maintenance annually, and leaves infrastructure, including bridge piers and docks in unsafe conditions. Scour occurs during times of rapid river flow and icing conditions when sediments, including rocks, gravel and silt are transported by the currents away from bridge piers and similar structures. If the event is severe enough, foundation material below the pier footing may be eroded, leaving the structure unsupported and in jeopardy of collapse. Scour is dynamic; ablation and deposition can occur during the same high-energy river event, so the net effect cannot be easily predicted.
While there are several current techniques available for measuring or monitoring scour, most have severe weaknesses. Radar has been employed successfully to bathymetrically determine scour conditions. The technique is usually applied after an event, indicating the final status of the sedimentation surrounding a pier. Sonar techniques have been similarly applied. Neither of these techniques are continuously employed in situ during a scour event; both require skilled operators to perform the test and interpret the results. Both methods are not particularly successful when there is rubble, ice or debris covering the water.
Neutral buoyancy “fish” equipped with a seismic transducer and radio transmitter have been anchored at varying depths in the sediments around bride piers (Zabilansky, 1996). As these “fish” are uncovered by the scouring process and are caused to move by the currents, their radio transmitter signals a receiver. on shore that the scour has reached their tethered depth. During a depositional event, the “fish” are re-buried at approximately their original depth. While this system is re-settable, it still gives a fairly crude spatial indication of the scour progression. The “fish” are battery powered and thus have a limited active life and must be replaced seasonally.
A time domain reflectometry technique has been suggested in the literature (Dowding and Pierce, 1994). This system uses a sacrificial sensor buried vertically in the sediments. Once a section of the sensor is exposed to scouring, the currents causes the exposed section to be broken off, shortening the sensor. This shortening of the sensor can be detected and measured with an on-shore instrument. The drawback of such a system is that the sensor is sacrificial, i.e. destroyed in the measurement process, and must be replaced after every event.
Several permanent and robust TDR-based scour monitor implementations have been disclosed and are in various stages of being patented by Yankielun and Zabilansky, these include: U.S. Pat. No. 5,784,338: “Time Domain Refectometry System for Real-Time Bridge Scour Detection and Monitoring”; and U.S. Pat. No. 5,790,471: “Water/Sediment Interface Monitoring System Using Frequency-Modulated Continuous Wave”. The time-domain technique has a preferred implementation, which utilizes an umbilical cable to communicate data to an on-shore measurement instrument and data acquisition, but can alternatively by implemented using wireless radio telemetry to communicate data. The FM-CW system has a preferred implementation that transmits data to a remote acquisition and analysis system via wireless radio telemetry, but can alternatively be implemented using a coaxial cable to transfer data. Both of these sensors are high-resolution (Approximately 2 cm) scour measurement devices. While these devices provide high-resolution data when used in sand and gravel based sediments, they are ineffective when used in consolidated (clay-based) and otherwise lossy soils. When emplaced in lossy soils the electromagnetic pulse that normally travels down and back the length of the sensor is severely attenuated in the soil. Thus, little or no pulse reflection from material boundaries (soil/water and end of probe) is detected. It is estimated that approximately half of all riverene sites have sediments in which the earlier prove implementation would be severely impacted or rendered completely useless.